Binders or binder systems for foundry cores and molds are known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern of the core or mold, and then cured to a solid using catalysts, heat, and/or a co-reactant.
Resin binders used in the production of foundry molds and cores are often cured at high temperatures to achieve the fast-curing cycles required in foundries However, resin binders have been developed which cure at low temperatures. These processes are preferred over high-temperature curing operations that have higher energy requirements and often emit undesirable fumes.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as no-bake processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern, either a core or mold. Curing of the binder is achieved without heating.
One such no bake process employs an aqueous alkaline solution of a phenolic resole resin as the binder. In this process, the foundry sand is usually mixed with an ester-curative before the solution of resole resin is added to the mixture. The process is described in detail in U.S. Pat. No. RE32,812 which is incorporated herein by reference in its entirety.
Another process which does not require the application of heat to cure a core or mold is the cold box ester-cured process. In this process, a foundry core or mold is prepared by mixing sand with an alkaline solution of a phenolic resole resin, discharging the mixture into a pattern, and curing the mixture by contacting the resin with a vaporous ester-curative. In the cold box ester-cured process, the phenolic resole resin may be cured with low molecular weight, gas phase, carboxylic acid esters, such as alkyl formates, including methyl formate and ethyl formate.
The ester cured process is superior to some of the earlier processes from an environmental standpoint. However, the initial tensile strengths of the cores and molds made by this process may be somewhat lower than those prepared by other binder processes.
In one prior art composition, in an attempt to improve the tensile strength of the cores and molds, the amount of phenolic resin present in the binder was increased, resulting in higher tensile strengths of the ester-cured resins. This was achieved by using a solution of a phenolic resin, either novolac or resole, in an organic ester-curative.
In yet another prior art composition, the compressive strength of cores made from an aggregate bound with a phenolic resole were improved by the incorporation of lightburned magnesia into the resin.
Compositions which accelerate the cure of an ester-cured phenolic resole are also disclosed in the prior art. However, accelerating the cure of a binder does not consistently result in an increase in the strength of the cured cores or molds.
In another prior art composition, a polyphenol is added to the phenolic resole resin in an attempt to improve the strength of a bound core or mold. The polyphenol resins are made by reacting a phenol or a substituted phenol with a peroxidase or an oxidase enzyme and peroxide in an organic solvent-containing medium.
Other attempts to improve the strength of cores and molds bound with an ester-cured phenolic resole have included the addition of a silane coupling agent to the aggregate or the binder. However, silane coupling agents are costly and do not always result in the desired strength improvement for a given application.
Each of the prior art compositions and methods for improving the strength of a core or mold bound with an ester-cured phenolic resole are not without disadvantages. Adding resin solids to the ester-curative has not always provided the needed strength improvement. Because the foundry binder systems are applied to an aggregate as liquid solutions, the use of a lightburned magnesia compound requires the dissolution of such compounds into the binder. This may represent a difficult step and an added cost to prepare such a solution. The use of a polyphenol requires the costly first step of preparing such a resin. The use of silane coupling agents also represent a significant added cost because such silanes themselves are costly and the use of a silane coupling agent does not always result in the desired strength improvement.
It would therefore be an advantage to have an ester-cured binder system that provides significantly stronger cores and molds. It would be yet a further advantage to have a method for improving the tensile strength of cores and molds bound with an ester-cured resin. It would be an even further advantage to provide an additional cross-linker to the cross-linking of alkaline phenolic resoles thereby providing additional strength to the cured resin.